One way drilling a borehole can be accomplished is by circulation of fluid through a downhole motor which is operably connected to the drill bit. Such bottomhole assemblies have, at times in the past, employed thrusters in an effort to improve drilling efficiency. The thruster is a telescoping tube arrangement which allows the drill bit to advance while the tubing string is supported in a rather stationary position at the surface. Ultimately, when the thruster has advanced its full stroke, or a notable portion thereof, the drill string is lowered from the surface, which causes the upper end of the thruster to slide down and therein close the thruster for the next stroke. When the drilling kelly or the stand being drilled down by the top drive reaches the drill rig floor, circulation is interrupted and another piece of tubing is added to the string at the surface or the coiled tubing is further unspooled into the wellbore. This also causes the thruster to retract as a result of this procedure and the drilling procedure using the downhole motor begins once again.
In the past, depending on drilling conditions, fluid resistance in the downhole motor varies as a result of torque generated at the drill bit which is connected to the drilling motor. Fluctuations of pressure drop through the motor caused by the above-noted bit torque change has in the past impeded the function of the thruster. What had occurred in the past was that the thruster responded to changes in pressure drop through the downhole motor instead of simply maintaining a fixed weight on bit as the drill bit advanced at a constant weight on bit (WOB). The inability of the thruster to maintain relatively constant weight on bit, regardless of the amount of work the drilling motor was required to do, caused instability to such thrusters to the point of negating their functional operation and negatively impacting the drilling operation. What occurred was a pressure increase due to higher torque load on the motor as a result of changing drilling conditions. The higher or increased pressure was sensed at the thruster, causing it to try to extend the telescoping portion out further, which in turn increased the WOB. Ultimately, with increasing WOB, the motor torque was greater and the pressure sensed by the thruster was therein greater and drilling would cease as the thruster drove the motor in a stall condition where the drill bit is no longer turning.
In these past applications of the thruster, the WOB was a function of the pressure difference between inside and outside the thruster. The greater the difference, the more force on the bit is exerted by the thruster. As a result, assemblies using thrusters with downhole motors in combination with drill bits have not been as efficient and useful as possible.
An object of this invention is to provide a pressure-modulation valve in the bottomhole assembly between the thruster and the downhole motor to compensate for pressure increases as a result of changing drilling conditions which have, in the past, caused an increase in torque and, as a result, winched the WOB applied by the thruster. Ultimately, it is the function of this invention to make a thruster operable when used in conjunction with the drilling motor so that it can efficiently and reliably, without undue cycling or oscillation, feed out pipe in response to advancement of the drill bit during the drilling operation. Use of the pressure-modulation valve facilitates a constant WOB since variations in pressure drop in the circulating mud in the drilling motor do not affect the relative force exerted on the bit. With the modulation feature fully effective, these variations in pressure drop are compensated by the pressure-modulation valve with the result being a facilitation of a constant WOB regardless of motor differential pressure.